1. Field of the Invention
The present invention relates to a nitride semiconductor device, more particularly, in which an active layer having an optimized structure of quantum barrier and well layers improves luminescence efficiency particularly in the case of high power operation.
2. Description of the Related Art
In general, nitride semiconductor devices, such as green or blue Light-Emitting Diodes (LEDs) and Laser Diodes (LDs), are widely used as a light source of full color displays, image scanners, various signal systems and optical communication devices. The nitride semiconductor devices can be provided as luminous devices that emit various colors of light such as green and blue light using an active layer based on electron-hole recombination.
After the development of the LEDs, a number of technical advancements have been achieved and widened their application fields. Accordingly, the LEDs are under study as general lighting devices and electronic devices. In particular, while conventional nitride light-emitting devices have been generally used as parts, which are applied to mobile products requiring low current and low power operation, the application fields are recently expanding into the fields requiring high current and high power operation. Accordingly, development of an LED structure having a high efficiency at high current operation is urgently demanded.
FIG. 1 is a cross-sectional view illustrating a typical nitride semiconductor device.
Referring to FIG. 1, a nitride semiconductor device 10 includes an n-type nitride semiconductor layer 12, an active layer 15 having a multiple quantum well structure, a p-type nitride semiconductor layer 17 and a transparent electrode layer 18, formed sequentially on a sapphire substrate 11. A portion of the n-type nitride semiconductor layer 12 is etched to provide an area on which an n-type electrode 19a is formed, and a p-type electrode 19b is formed on the transparent electrode layer 18. The active layer 15 has a multiple quantum well structure consisting of a plurality of quantum well layers 15a and a plurality of quantum barrier layers 15b, which are alternately stacked on each other.
The luminescence efficiency of the nitride semiconductor device is generally determined by internal quantum efficiency, the probability of electron-hole recombination in the active layer. Attempts to improve the internal quantum efficiency are subjected to researches, generally for the purpose of increasing the number of effective carriers participating in light emission by improving the structure of the active layer. In other words, it is required to decrease the number of effective carriers overflowed from the active layer in order to increase the number of effective carriers in the active layer.
In addition, since carriers can be injected to only a specific local area of the active layer, the effective light-emitting area in the entire active layer is limited. Since this limitation of the effective light-emitting area is directly connected with degradation in luminous efficiency, an attempt capable of ensuring electron-hole recombination in the entire active area is demanded. This will be described in more detail with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are graphs of simulation results illustrating the distribution of a carrier wave function and an effective active area with respect to active layers, in which seven pairs of quantum well layers and quantum barrier layers are formed with thicknesses 30 and 150 Å, respectively, as examples of a conventional nitride semiconductor device.
Firstly, referring to the wave function (dotted line: electrons, solid line: holes) shown in FIG. 2A, the probability of existence of the holes sharply decreases with the number of the pairs increasing since the holes are relatively less mobility than the electrons. While the distribution of the electrons and the holes decreases as the electrons and the holes are more remote from n-type and p-type nitride semiconductor layers, the number of the holes relatively more sharply decreases. Accordingly, as shown in FIG. 2B, effective recombination probability tends to be higher in a quantum well layer located in an area II, which is more adjacent to the p-type nitride semiconductor layer.
Such a problem of efficiency droop or reduction in the effective active area of the active layer is more prominent especially when the semiconductor device is used in a lighting device that requires high current operation. Accordingly, a multiple quantum well structure capable of improving luminous efficiency in high power operation is required in the art.